Serve JS libs with static templatetag instead of hardcoded url

[pierreben]

I found a small issue in the default config where the static URLs are currently hardcoded :

    # Defaults to the bundled django-altcha file.
    "ALTCHA_JS_URL": "/static/altcha/altcha.min.js",
    # URL of the Altcha translations JavaScript file.
    # Defaults to the bundled django-altcha file.
    "ALTCHA_JS_TRANSLATIONS_URL": "/static/altcha/dist_i18n/all.min.js",

To address this, I implemented a lazy static URL loader, which allows the use of Django's static template tag within this configuration.